Updated

A vocal critic of Russian President Putin’s government slipped into a life-threatening coma caused by an unknown poison, his wife said, according to The New York Times.

Vladimir Kara-Murza was hospitalized last week after becoming ill. Kara-Murza, a journalist and a close associate of the murdered opposition leader Boris Nemtsov, nearly died when he suffered sudden kidney failure in May 2015.

No cause for that illness has been determined, but Kara-Murza underwent tests showing he had ingested a poisonous substance.

In light of the fatal poisoning of defector Alexander Litvinenko and the mysterious deaths of other Russian opposition figures, some worried that Kara-Murza could have been deliberately poisoned.

Doctors have been keeping him alive on artificial respiration, the report said.

“The onset was just as sudden as last time,” Yevgeniya Kara-Murza said. “He is all surrounded by machines and covered in tubes.” Other than poisoning, she said, “We don’t see any other explanation.”